David G. Wilder

Epidemiologic Studies of Low-back Pain

The records of 3920 patients (2068 females, 1852 males) entering a model family practice unit between 1975 and 1978 have been analyzed. Eleven percent of males and 9.5% of females reported an episode of low-back pain during that 3-year interval. The complaint of medically reported low-back pain was significantly related to occupational factors such as truck driving (P<0.001), lifting, carrying, pulling, pushing, and twisting (P<0.001 for all variables) as well as nondriving vibrational exposure (P<0.001). Patients reporting low-back pain also reported more episodes of anxiety (P<0.001) and depression (P<0.001) and had more emotionally stressful occupations (P<0.001). The mean number of pregnancies was greater in women with low-back pain (2.6) than in those without (1.6) (P<0.001). The low-back pain sufferers were more likely to be cigarette smokers (P<0.001), particularly when smoking was accompanied by a chronic cough (P<0.001). This population is currently under prospective study to define the relevance of each of these risk factors to the complaint of low-back pain.

Muscular Response to Sudden Load: A Tool to Evaluate Fatigue and Rehabilitation

Study Design Subjects were exposed to fatiguing and restorative interventions to assess their response to sudden loads. Objectives To investigate the erector spinae and rectus abdominis response characteristics to “sudden load” and the effect of fatigue and rehabilitation. Summary of Background Data Unexpected loads, which people often experience, can lead to high forces in the spine and may be a cause of low back injury. Methods Muscle responses to sudden load were mediated by fatigue, walking, expectation, method of load application, exposure to vibration, and cognitive‐behavioral rehabilitation in patients with chronic low back pain. A novel technique, perfected in this work, called wavelet analysis, was used to analyze these data. Results Reaction time was affected by fatigue and expectation. Vibration exposure significantly increased the muscle response time. Walking was able to ameliorate that effect. Back muscles responded differently, depending on whether loads were applied to the back through the hands or through the trunk. Electromyographic reaction time and magnitude decreased in patients after a 2‐week rehabilitation program. Conclusions Sudden loads can exacerbate fatigue effects. Walking after driving reduces the risk to the back caused by handling unpredictable loads. Vibration exposure guidelines should be more conservative. Patients have longer response times than healthy subjects, but patients can improve their response to sudden loads via rehabilitation. Patients exhibit a flexion‐extension oscillation at 5 Hz in response to a sudden load, suggesting that the 5‐Hz, seated, natural frequency observed during whole‐body vibration may result from neurophysiologic control limits.

Load-sharing between anterior and posterior elements in a lumbar motion segment implanted with an artificial disc.

Study Design. A nonlinear three-dimensional finite element model of the osteoligamentous L3–L4 motion segment was used to predict changes in posterior element loads as a function of disc implantation and associated surgical procedures. Objectives. To evaluate the effects of disc implantation on the biomechanics of the posterior spinal elements (including the facet joints, pedicles, and lamina) and on the vertebral bodies. Summary of Background Data. Although several artificial disc designs have been used clinically, biomechanical data—particularly the change in loads in the posterior elements after disc implantation—are sparse. Methods. A previously validated intact finite element model was implanted with a ball-and-cup–type artificial disc model via an anterior approach. The implanted model predictions were compared with in vitro data. To study surgical variables, small and large windows were cut into the anulus, and the implant was placed anteriorly and posteriorly within the disc space. The anterior longitudinal ligament was also restored. Models were subjected to either 800 N axial compression force alone or to a combination of 10 N-m flexion–extension moment and 400 N axial preload. Implanted model predictions were compared with those of the intact model. Results. Facet loads were more sensitive to the anteroposterior location of the artificial disc than to the amount of anulus removed. Under 800 N axial compression, implanted models with an anteriorly placed artificial disc exhibited facet loads 2.5 times greater than loads observed with the intact model, whereas posteriorly implanted models predicted no facet loads in compression. Implanted models with a posteriorly placed disc exhibited greater flexibility than the intact and implanted models with anteriorly placed discs. Restoration of the anterior longitudinal ligament reduced pedicle stresses, facet loads, and extension rotation to nearly intact levels. Conclusions. The models suggest that, by altering placement of the artificial disc in the anteroposterior direction, a surgeon can modulate motion-segment flexuralstiffness and posterior load-sharing, even though the specific disc replacement design has no inherent rotational stiffness.

Are occupational drivers at an increased risk for developing musculoskeletal disorders

Study Design This study analyzed the role of exposure to driving and other covariates in reports of back, neck, and shoulder pain and resultant disability. Cohorts in Sweden and the United States were compared. Objectives To establish the effect of mechanical and psychosocial factors in reporting back, neck, and shoulder pain and work loss. Summary of Background Data There are numerous reports of a positive relationship between back pain and driving. However, exposure data are minimal. The influence of job satisfaction has not been assessed. Methods The physical factors affecting reports of back, neck, and shoulder pain were investigated in a two‐country cohort study of bus and truck drivers and sedentary workers. Vibration exposure was obtained by directly measuring the vibration imposed on the driver during a typical work day. Lifting exposure was attained by questionnaire. Cumulative exposure was computed based on work history. Musculoskeletal health information was based on a modified nordic questionnaire, and other questionnaires recorded the physical and psychosocial aspects of the work environment. Results Of the sample, 50% reported low back pain, with no difference between countries. The highest risk factors (odds ratios) for back and neck pain were long‐term vibration exposure, heavy lifting, and frequent lifting. A combination of long‐term vibration exposure and frequent lifting carried the highest risk of low back pain. Work loss from low back pain was influenced by perceived job stress. Conclusions Vibration (resulting from driving) and lifting cause back, neck, and shoulder pain, whereas inability to work seems affected by stress at work.

The relationship between anthropometric, postural, muscular, and mobility characteristics of males ages 18-55

Three hundred twenty-one males, ages 18–55, had standardized tests to determine height, weight, Davenport Index, leg length inequality, determination of flexion and extension torques, flexion/extension balance, range of motion, straight leg raising, and lumbar lordosis. A total of 106 (33.0%) had never experienced low-back symptoms; 144 (44.9%) had or were having moderate low-back pain (LBP); and 71 (22.1%) had or were having severe low-back symptoms. These three subgroups showed no significant differences in height, weight, Davenport Index, lumbar lordosis, or leg length inequalities. LBP patients had less flexor and extensor strength and were flexor over-powered, had diminished range of motion for spinal extension and axial rotation (P = 0.003, P = 0.0005), and diminished straight leg raising capacity (P = 0.04). A multivariate correlation matrix demonstrated no typical pattern of associated abnormalities except a diminished spinal range of motion in one plane was associated with the anticipated diminishment in all other planes of motion, and often with greater restrictions of straight leg raising tests.

1980 Volvo award in clinical sciences. Assessment of patients with low-back pain by biplanar radiographic measurement of intervertebral motion.

Abnormalities of intervertebral joint motion including hypermobility, reduced mobility, torsional abnormality, and displacement of the center of rotation have been associated with degenerative change. However, measurement of these signs in plane X-ray films is handicapped by the three-dimensional motion and geometry of the spine. This study aimed to relate three-dimensional motion of the joints to their pathological state. We have used biplanar radiography to measure intervertebral motion during voluntary movements by patients with low back pain. Primary (or intentional) and coupled motions were measured by a refined technique, along with disc shear and facet joint motion. Abnormalities were found, especially in the “coupled” motions which were related to narrowed disc space, and to proximity to previous fusions. There was asymmetry of motion specific to joints with herniated nucleus pulposus.

European Spine Society--the AcroMed Prize for Spinal Research 1995. Unexpected load and asymmetric posture as etiologic factors in low back pain.

Unexpected loads, which often occur in the working environment, can lead to high forces in the spine and, thus, may be a cause of low back injury. This paper discusses the effect of “sudden load” on the erector spine reaction and amplitude. Muscle responses were mediated by several factors, including fatigue, posture, expectation and rehabilitation, in chronic low back pain patients. The subjects were fatigued by holding a 20% maximum voluntary contraction for 1 min. A functional restoration program was tested for its efficacy in reducing reaction time and EMG amplitude in chronic low back pain patients. Reaction time was longer and EMG amplitude lower in patients than in their matched controls. EMG reaction time and magnitude decreased in patients after a 2-week rehabilitation program, including specific training of coordination and posture control. The results of the modelling showed higher spinal compressive load and lower shear forces when the load was expected than when the load was unexpected. The effect of sudden loads can be exacerbated if a worker is not standing on a flat surface or is fatigued. Chronic low back pain patients have less ability to protect themselves from sudden loads, but they can be trained to improve their response by means of an appropriate rehabilitation program.

Vibration and the Human Spine

Vibrational effects have been found to be associated with increased frequency of low-back pain in various industries, and because of this the study herein reported was conducted to assess the physiologic response of the spinal system to sinusoidal vibrations. A device replicating industrial vibration was constructed and employed on healthy human volunteers to determine the stiffness, impedence, and resonant characteristics of the subjects. Three peaks of enhanced transmissibility corresponding to the resonant frequency of the spinal system were found. The greatest transmissibility of vibatory input occurs at the first resonant frequency (5Hz) where marked enhancement of vibrational input occurs as vibrations pass through the spinal system. A progressive stiffening of the system occurs in response to vibrational inputs of increasing frequency. The effects of posture, the Valsalva maneuver, and fatigue alter the normal response. Structures vibrated at the first resonant frequency have greater potential for damage.

The Biomechanics of Lumbar Disc Herniation and the Effect of Overload and Instability

A multipart study has been performed to provide a mechanical explanation for the epidemiologic association between sitting in static (e.g., factory or office) or vibration (e.g., car or truck driving) environments and acute herniated lumbar discs. It was shown that a 1 h exposure to sitting environments caused significant changes in the mechanical properties of the lumbar intervertebral disc. During many of the latter tests, specimens were unstable (exhibited by a sudden, large flexion and/or lateral bend rotation response to an axially applied load). This showed that a motion segment in the lumbar spine could suddenly buckle and apply a tensile impact loading to the posterolateral region of the disc. We also demonstrated that a combined lateral bend, flexion, and axial rotation vibration loading could cause tracking tears proceeding from the nucleus through the posterolateral region of the anulus. It suggests that a mechanism for disc herniation is mechanical changes leading to instability of the motion segment. These experiments complete the argument that lumbar disc herniations can be a direct mechanical consequence of prolonged sitting in static or vibration environments.

Epidemiological and aetiological aspects of low back pain in vibration environments—an update

The article reviews the substantial body of epidemiological evidence linking vibration exposure and low back pain. Drivers appear to be at particular risk if exposures exceed those recommended by the ISO exposure limit. Various aetiological factors associated with vehicular vibration, flattening of the lumbar lordosis, increased motion segment flexibility, disc pressure and mechanical softening are discussed. Vibration studies of functional spinal units are also discussed, as are in vivo whole-body vibration experiments. Animal models have shown that vibration leads to compromised nutrition, higher disc pressures, release of neuropeptides, increased creep and histological changes.